Free SO2 endpoint measurement accuracy

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Junior Member
Jan 24, 2016
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This was my first experience using the morewinemaking aeration kit.* I have 6 gallons of a Winexpert kit ready to bottle and thought this would be a good opportunity to learn the Free SO2 measurement process.

The instructions were easy enough to follow, but I am wondering about the rate of aeration.* The kit recommends aerating for 10 to 15 minutes. I arrayed for 15 minutes and added 0.4ml indicating 6.4ppm

Seemed lower than it should, so I aerated for another 15 minutes.* Added 0.6ml more for a total of 1.0ml NaOH.* Indicating 16 ppm.

I aerated one more time.* The color change from heat back to pink was slower so this time went for 30 minutes.* Then added 0.4ml more for a total of 1.4ml NaOH.* Indicating*22.4 ppm.

So the aeration time affects the result.* Or do I not have enough air flow for the bubbles?

Can one aerate too long?
I have not used that specific setup, but something doesn't seem right as you shouldn't have to run that long. Be careful that you are not getting wine sample (mist) to carry over to the peroxide trap. In other words, the tubing from the wine sample flask to the peroxide flask should remain dry during the test. Any sample carry-over will cause the results to continue to climb as you describe. As long as you're getting a liter of air per minute, I would stick with the 10 to 15 minute recommended time frame.
I have this setup and it works great. Assuming you have fresh chems and have adjusted your receiving flask with HCL so that it is right at the endpoint to begin with as long as you have a nice steady bubble flow (not crazy heavy bubble flow) and let it go for 15 mins you have pushed everything over to the receiving flask for titration. Trust the results and add KMETA using the handy SO2 calculator in Winemaker magazine using your found results, your pH and your required SO2 from the calculator.

Watch the youtube videos if you are unsure of anything. This is an endpoint titration using your eye so you need to calibrate your eye for just when it starts to turn grey/green.

This is the defacto standard for free SO2 analysis in the wine industry so trust the results.
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Thanks for the reply. I am thinking I might not have been pushing enough air. There wasn't any mist in the transfer tube. I was only getting a few bubbles a second. Trying to think how I can verify I am getting a litre of air per minute.

Got any ideas?
If you feel you really need to verify the flow, a down and dirty way to get a rough estimate is to take a 1 liter bottle, submerge bottle into a bucket of water and allow bottle to fill with water, invert bottle while keeping the opening under water, turn on the pump and put the air outlet tube into the inverted bottle allowing the bubbles to be collected in the bottle, the bottle should be full of air in one minute.
Don't overthink this. Nice steady flow. 15 minutes. The key here is to be consistent on everything especially endpoint determination.
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I agree with Mike, if you follow the directions you should be ok, but I posted the information just in case you are the type that will lose sleep without verification.
It should go without saying that you need to make sure your (tubing) connections, stoppers etc. are all on tight. The free SO2 gets driven of as a gas so once you add the Phosphoric acid get the stopper on ASAP. As the gas passes through the receiving solution, the Hydrogen Peroxide converts (and traps) the SO2 to Sulfuric Acid. You are then titrating that solution back to the endpoint (grey/green color change) using the NaOH.
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